LibCrypto: Fix issues in the Crypto stack

This commit fixes up the following:
- HMAC should not reuse a single hasher when successively updating
- AES Key should not assume its user key is valid signed char*
- Mode should have a virtual destructor
And adds a RFC5246 padding mode, which is required for TLS

Libraries/LibCrypto/Cipher/Cipher.h

@@ -40,6 +40,7 @@ namespace Cipher {
 
     enum class PaddingMode {
         CMS, // RFC 1423
+        RFC5246, // very similar to CMS, but filled with |length - 1|, instead of |length|
         Null,
         // FIXME: We do not implement these yet
         Bit,